Study on Variable Speed Wind Turbines’ Capabilities for Frequency Response
International Journal of Engineering RESEARCH · Although the use of variable speed wind turbines...
Transcript of International Journal of Engineering RESEARCH · Although the use of variable speed wind turbines...
IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589
Please cite this article as: E. Jamila, S. Abdelmjid, Comparative Study of the Performance of Static Synchronous Compensator, Series Compensator and Compensator /Battery Integrated to a Fixed Wind Turbine, International Journal of Engineering (IJE), TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589
International Journal of Engineering
J o u r n a l H o m e p a g e : w w w . i j e . i r
Comparative Study of the Performance of Static Synchronous Compensator, Series
Compensator and Compensator /Battery Integrated to a Fixed Wind Turbine E. Jamila*a, S. Abdelmjidb
a Mechanical Engineering Laboratory, Faculty of Sciences and Technology FST, Road Immouzer, Fez, Morocco b Engineering, systems and applications Laboratory, National School of Applied Sciences of Fez (ENSA)
P A P E R I N F O
Paper history: Received 31 August 2015 Received in revised form 14 April 2016 Accepted 14 April 2016
Keywords: Fixed Speed Wind Turbine Static Synchronous Compensator Low Voltage Ride Through Capability Static Synchronous Series Compensator Static Synchronous Compensator /Battery
A B S T R A C T
This paper studies the interest of the integration of battery energy storage with Static Synchronous
Compensator (STATCOM) for improving the low voltage ride through capability (LVRT) of a fixed
speed wind turbine connected to the grid. For this reason and by applying a grid fault, a comparison is
made between integrating the SSSC, the STATCOM and the STATCOM with battery energy storage.
The system with the aforementioned flexible alternating current transmission system (FACTS) systems
is simulated using MATLAB/SIMSCAPE and the results show that the STATCOM with a battery is
most efficient in terms of improving the LVRT of a fixed speed wind turbine.
doi: 10.5829/idosi.ije.2016.29.04a.18
1. INTRODUCTION1
The penetration of wind power into the electric power
systems is in constant growth, which presents a great
challenge to meet the network requirements. Indeed,
with the increasing share of energy from the wind
power system, wind turbines have to remain connected to the grid in transient voltage conditions in order to be
in compliance with the grid codes which vary according
to the transmission system operator (TSO) [1, 2]. The
grid requires that wind turbines be treated as
conventional production units and participate in the
regulation of active and reactive power and voltage
control, and system frequency [3].
In this paper, we examine the ability of a fixed speed
wind turbine to gather under voltage grid requirements
and stabilize the grid voltage during faults by applying
the FACTS devices. The performance of this system with either the STATCOM system compensating device
or the SSSC has been shown in several studies but the
objective of this study is to compare the performance of
1*Corresponding Author’s Email: [email protected] (E. Jamila)
the STATCOM, SSSC and the STATCOM with a
battery energy storage.
For this purpose, we model in
MATLAB/SIMSCAPE, the system under grid fault with
firstly the STATCOM, the SSSC and finally with
STATCOM/battery energy storage. These FACTS
devices are evaluated for their performances in terms of under voltage requirements and from the simulation
results, a comparison of their performances is presented.
2. SYSTEM PRESENTATION The system consists of a fixed speed wind turbine
connected to the grid. The FACTS device (STATCOM,
SSSC or STATCOM with battery) is connected between
the wind turbine and the grid at the point of common coupling PCC (Figure 1) in order to improve power
quality during faults.
2. 1. Wind Turbine Wind turbines are systems that
harness the kinetic energy of the wind for useful power.
Wind flows over the rotor of a wind turbine cause
RESEARCH
NOTE
E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589 582
Figure 1. SCIG connected to the grid
it to rotate on a shaft. The resulting shaft power can be used for mechanical work, like pumping water, or to
turn a generator to produce electrical power [4].
For converting kinetic energy into electrical energy,
two different approaches exist, fixed speed and variable
speed [5]. The fixed speed wind turbines using squirrel-
cage induction generators (SCIGs) are the traditional
ones while the variable speed wind turbines have
received increasing attention during the past decades.
Although the use of variable speed wind turbines is the
trend, fixed speed wind turbines are still used widely
especially in offshore wind farms [6, 7]. In this work, the fixed speed wind turbine using
squirrel-cage induction generator is used. During a fault,
squirrel-cage induction generator will accelerate due to
the imbalance between the mechanical power extracted
from the wind and the electrical power delivered to the
grid. After a fault, the generator consumes reactive
power and it slows down voltage restoration. When the
voltage does not rise quickly enough, the generator
continues to accelerate and consume even larger amount
of reactive power. This process may eventually lead to
voltage and rotor speed instability, if the wind turbine is
connected to a weak grid. To prevent these types of instabilities, the fact devices can be connected to the
system.
2. 2. STATCOM The Static Synchronous
Compensator (STATCOM) is a switching converter-
based, shunt connected device used to regulate voltage
and power flow on electric power systems by means of
reactive power injection or absorption [8]. This device
consists of a voltage-sourced converter (VSC), which
generates a controlled AC voltage at its output, with the use of a coupling transformer, DC capacitor and power
electronics [9]. The converter is controlled by PWM
techniques (Figure 2).
2. 3. SSSC The Static Synchronous Series
Compensator (SSSC) is one of important FACTS
devices.
Figure 2. STATCOM control scheme [9]
It is a voltage source converter which injects, from a DC
voltage source (capacitor), an almost sinusoidal voltage
of variable and controllable amplitude and phase angle,
in series with a transmission line [10].
The injected voltage is almost in quadrature with the
line current in order to increase or decrease the overall
reactive voltage drop across the line and thus control the
reactive power flows.
The control system (Figure 3) implements a PWM
that generates the switching signals for the VSC from
the calculated d and q components of the converter voltage [9].
2. 4. STATCOM/Battery Without an energy
storage system, FACTS devices can support the grid
with only reactive energy. By integrating an energy
storage system (ESS) with FACTS devices, an
independent real and reactive power absorption or
injection into and from the grid is possible. Since the STATCOM is the most important FACTS
device, the combination of STATCOM with ESS
enables significant performance improvements over traditional STATCOM. In fact, with this combination, it
is possible to control also the active power flow
between the STATCOM and the grid unlike the
STATCOM without ESS which allows an exchange of
reactive energy only.
Figure 3. Systematic diagram of SSSC
583 E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589
The ability to control the active power and reactive
power is highly effective especially for damping rapidly
the oscillations and responding to sudden load
transients.
The combination STATCOM with ESS is realized
by integrating an ESS to the DC bus of the STATCOM. The ESS used are: Superconducting Magnetic Energy
Storage (SMES), Flywheel Energy Storage (FES),
Advanced Capacitors and Battery Energy Storage. The
Battery Energy Storage is the best suited in STATCOM
since it rapidly injects or absorbed reactive power to
stabilize the grid system. It also controls the distribution
and transmission system in a very fast rate.
3. SYSTEM MODELING IN MATLAB/SIMSCAPE The wind energy generating system is connected to the grid through a transmission line of 25 km. We apply a
fault to the system in order to measure the performance
of the different FACTS devices: STATCOM, SSSC and
STATCOM/battery. So, we have three topologies to
model in MATLAB/SIMSCAPE.
The model of the system with the STATCOM is
illustrated in Figure 4. The detailed model of the
subsystem ‘wind turbine’ is illustrated in Figure 5.
Figure 4. Model in SIMSCAPE of the wind turbine with STATCOM during fault
Figure 5. Model in SIMASCAPE of the wind turbine system
E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589 584
3. 1. Model of the System with STATCOM The
block ‘ wind couple’ shown in Figure 8, calculates the
aerodynamic torque applied to the blades. This torque
depends on the wind speed, the speed of the generator and the pitch angle [4] as shown in Equation (1):
Pm=1/2.ρ.pi.R².v³.Cp(λ,β) (1)
Cp is the power coefficient or performance coefficient
which indicates the efficiency with which the turbine converts the mechanical energy of the wind into
electricity. The theoretical maximum coefficient of
performance, 16/27, is never achieved by practical wind
turbines due to the irregularities in the wind speed and
other environmental factors. A more realistic value for
the Cp for existing wind turbines ranges from 30-50%
[11].
This coefficient differs according to the turbines. In
our case, the coefficient is given by the relation in
Equation (2). It is the most used formula.
Cp=0.22(116/λ’-0.4a–5)*exp(-12.5/λ’) (2)
where a is the angle of attack of wind turbine and λ’
depends on λ and a as shown in Equation (3):
1/λ’=1/(λ+0.08a)-0.035/(a³+1)ρ (3)
and λ is the specific speed witch is calculated by
Equation (4):
λ=U/v=ω.R/v (4)
where U is the tip speed of the blades, v is wind speed, R is the radius or length of the blades and ω is the
rotational frequency of the rotor (in rad/s). β is the pitch
angle of the blades
The blocks ‘STATCOM’ and ‘FAULT’ are ready to
use blocks in MATLAB/SIMPOWERSYSTEMS. The
block ‘FAULT’ applys a three phase short circuit where
the opening and closing times can be controlled either
from an external Simulink signal (external control
mode), or from an internal control timer (internal
control mode)2.
3. 1. 1. Model of Wind Couple Subsystem The
calculation of the wind couple is based on Equation (1)
which is linearized to form a transfer function. 3. 1. 2. Model of Rotor Subsystem in SIMMECHANICS In Figure 7, the blocks bodies
model the blades and the rotor bodies (specified by their
masses, inertia tensors, and attached body coordinate
systems (CSs)) and the blocks joints represent possible motions of bodies relative to one another. The rotor and
blades can rotate relative to ground. Also, the blades can
make a rotational movement relative to the hub to vary
thereby the pitch angle [4].
2 www.mathworks.com
3. 1. 3. Model of Gear Train In the model below
(shown in Figure 8), simple and planetary gears are used
to transfer torque up and down the driveline axes. The
inertia block represents a rotating body specified by its moment of inertia. The choice of planetary gear and
simple gear is made to have a greater gear ratio [4].
Figure 6. Wind couple model
Figure 7. Rotor model
Figure 8. Model of gear train
2Tw1
1pitch_angle
CS4
CS5
CS6
CG
RotorBF
Revolute4
BF
Revolute3
B F
Revolute2
BF
Revolute1
Env
Machine
Environment
Joint Actuator2
Joint
Sensor2
Joint
Sensor1
Joint
Actuator4
Joint
Actuator3
Joint
Actuator1
Ground1
CS
1
Blade3
CS
1
Blade2
CS
1
Blade1
4Tw
3pitch2
2pitch1
1pitch
585 E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589
3. 1. 4. Model of the Asynchronous Machine The model illustrated in Figure 9 consists of an
asynchronous machine block ready to use and the loop
speed command of the generator based on Equation (5):
J dΩm/dt = ᴦe –fΩm-ᴦr (5)
3. 1. 5. Model of Turbine Protectio Protection
system (modeled in Figure 10 below) permits
immobilizing the wind turbine when the wind reaches a
certain strength or when the wind speed is below a
certain value by changing the pitch angle of the blades
for zero engine torque (beta=0) (shown in Figure 9). Also, in case of over speed generator, it allows
triggering the breaker in order to disconnect the wind
turbine from grid (shown in Figure 11) and varying the
pitch angle to 0 [4]. For the blade protection, if the wind speed is less
than 4 m/s or above 25 m/s for a time of 0.05 seconds,
the pitch angle value switches from 10 to 0. In order to
protect the generator, we switch the pitch angle to 90
and trigger the circuit breaker if the generator’s speed
exceeds the rated speed by 20% for a period of 0.05
seconds [4].
3. 1. 6. Model of Pitch Angle Command Block In the block depicted in Figure 12, the pitch command
output of the blade protection block switches on or off
the DC motor in order to vary the pitch angle by
rotating the blade.
Figure 9. Models of blade protection
Figure 10. Turbine protection model
Figure 11. Models of generator protection
Figure 12. Pitch angle command block
In our case, to pass from beta=10 to beta=0, we need
0.945s and then we stop again the motor. 0.945s is
needed because the rotational speed is equal to 30
tr/min. 1400 N.m/s is the value of the load torque
represented by the weight of the blade. For our wind
turbine, the weight of the nacelle and the rotor is equal
to 123 tons.
3. 1. 7. Model of Braking System Block The
block depicted in Figure 13, models the
hydromechanical command applied to the disc brake.
When we want to stop the wind turbine if the wind
speed reaches maximum speed, the hydraulic command
provides some pressure sliding thereby a cylinder which
in turn pushes the plates of the disc. The other end of
the disc brake is mounted on the rotor shaft.
For restarting, the input ON commands the flow of
the fluid in the opposite direction. So, the double acting
cylinder returns to its initial position.
The value of the applied pressure is calculated based
on the following dynamic equation:
* * / * 0 t P S J t (6)
ɷ0 is the rotational speed of rotor and S is the surface of
brake pad which is equal to 1.8 m in our case.
The duration of braking is equal to 3s, then ɷ (t = 3)
= 0, J is the moment of inertia of the rotor. It is
calculated by equating the rotor to a hollow cylinder and
considering the weight equal to 56 tons.
3. 2. Model of the System with Sssc In this case,
we consider the same previous model but instead of the
STATCOM, we use the block ‘SSSC’. This block
located in SimPowerSystems library, is connected in series between the wind turbine and the grid unlike the
STATCOM (Figure 14). 3. 3. Model of The System with STATCOM/Battery The model in SIMACAPE of the system with the SSSC
device is illustrated in Figure 15.
E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589 586
Figure 13. Braking system block
Figure 14. Model in SIMSCAPE of the wind turbine system with SSSC during fault
The block diagram of the main ‘STATCOM / Battery’
is shown in Figure 16. Unlike a STATCOM block, a
battery is used for energy storage. Thus, this block has
the advantage of storing and injecting active power and reactive power into the grid. The control of this FACTS
device is modeled by the block ‘control’ shown in
Figure 17 and Figure 18 which represents the block
‘Régulation_tension’, provides control of the voltage to
ensure the desired response from the system during
transient periods.
Figure 15. Model of the wind turbine system with STATCOM/battery during fault
Figure 16. Model in SIMSCAPE of the block STATCOM/battery
Figure 17. Block diagram of STATCOM/battery control
Figure 18. Model in SIMSCAPE of the block STATCOM/battery
Also, it converts this command PWM switching signals
to the STATCOM / battery.
4. SIMULATION RESULTS In order to compare the performance of adding the
STATCOM, the SSSC and the STATCOM/battery in
terms of improving LVRT capability of the wind
turbine, a simulation of the above models is made.
The technical data of the wind turbine, induction
generator, the STATCOM, the SSSC and the
STATCOM with battery are illustrated in Tables 1 to 3.
A three-phase fault is applied between times t = 0.5 s
and t = 1.1 s.
Figure 19 shows the voltages at the output of the turbine for different cases. It is clear that the addition of a
battery STATCOM improves the voltage during the
fault and quickly eliminates voltage fluctuations at
times of activation and deactivation of fault compared
with only STATCOM and SSSC (Figure 20 and Figure
21), and because of the exchange capacity of active
587 E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589
power flow. But the three FACTS components allow to
satisfy requirements under voltage.
As depicted in Figure 20, we compare between the
performance of the STATCOM and the SSSC. The
latter is more effective. It enables stabilizing the voltage
to 0.75 p.u during the fault and provides less voltage fluctuations in the moments of fault activation and
deactivation.
During the fault occurrence, the active power is set
to zero and the generator consumes a large amount of
reactive power. But with FACTS systems, reactive
power consumption is reduced as depicted in Figure 22.
We observe that the SSSC is more efficient for reactive
power compensation.
Figure 19. Voltage variation of wind turbine system during a three-phase fault and with STATCOM, SSSC and STATCOM
/battery
Figure 20. Voltage variation of wind turbine system during a
three-phase fault with STATCOM and SSSC
Figure 21. Comparing voltage variation of wind turbine system during a three-phase fault with STATCOM/battery and SSSC
As depicted in Figure 23, the STATCOM with a
battery is the only FACTS device which injects an
active power into the gird during the fault occurrence.
Figure 22. Reactive power consumption of the wind for a three-phase fault without a FACTS system and with STATCOM systems STATCOM and SSSC / battery
Figure 23. Model in SIMSCAPE of the block STATCOM/battery
TABLE 1. Technical data of the wind turbine system
Rotor
Tower hight 55m
Diameter 32m
Cut-in wind speed 4 m/s
Cut-out wind speed 27 m/s
Rated wind speed 13
Genarator
Rated power 275 kW
Rated voltage 600 V
Rated frequency 50 Hz
Stator resistance (Rs) 0.016 Ω
Stator inductance (Ls) 0.06 H
Rotor resistance (Rr) 0.015 Ω
Rotor inductance (Lr) 0.06 H
Pair of pole number 2
TABLE 2. STATCOM technical data
Power 200 kVA
Line series inductance 0.05 mH
Inverter Tension DC=800V / capacitance
DC=1875μF
Battery 1500 Ah
E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589 588
TABLE 3. SSSC technical data Power 200 kVA
Line series inductance 0.06 mH
Inverter Tension DC=800V / capacitance
DC=1875μF
5. CONCLUSION In this paper, we studied the interest of integrating a
battery energy storage to the STATCOM device for
improving the low votage ride through capability
(LVRT) of a fixed speed wind turbine. For this reason, a
comparison is made between the SSSC, the STATCOM
and the STATCOM with battery.
We modeled in MATLAB/SIMSCAPE the system consisting of the wind turbine connected to the grid and
with various FACTS devices. By applying a three-phase
fault, the simulation results show that the STATCOM
with a battery energy storage is most efficient in terms
of improving the LVRT capablity of the wind turbine.
6. REFERENCES
1. Iov, F., Cutululis, N.A., Hansen, A.D. and Sørensen, P., "Grid
faults impact on the mechanical loads of active stall wind
turbine", in International Symposium on Electrical and Electronics Engineering, ISEEE08. (2008).
2. Lipnicki, P. and Stanciu, T.M., "Reactive power control for
wind power plant with STATCOM ", in, Institute of Energy Technology, (2010).
3. Molinas, M., Vazquez, S., Takaku, T., Carrasco, J., Shimada, R.
and Undeland, T., "Improvement of transient stability margin in
power systems with integrated wind generation using a
STATCOM: An experimental verification", in International conference on future power systems. (2005), 16-18.
4. Jamila, E. and Abdelmjid, S., "Physical modeling of a hybrid
wind turbine-solar panel system using simscape language
(research note)", International Journal of Engineering-
Transactions B: Applications, Vol. 27, No. 11, (2014), 1767-1776.
5. Dominguez Garcia, J.L., "Modeling and control of squirrel cage
induction generator with full power converter applied to
windmills", (2009).
6. Banos, C., Aten, M., Cartwright, P. and Green, T., "Benefits and
control of STATCOM with energy storage in wind power
generation", in AC and DC Power Transmission, 2006. ACDC
2006. The 8th IEE International Conference on, IET. (2006),
230-235.
7. Hossain, M.J., Pota, H.R., Ugrinovskii, V. and Ramos, R.A., "A
robust STATCOM control to augment lvrt capability of fixed
speed wind turbines", in Decision and Control, 2009 held jointly
with the 2009 28th Chinese Control Conference. CDC/CCC
2009. Proceedings of the 48th IEEE Conference on, IEEE., (2009), 7843-7848.
8. D.Manasa, M.GopiSivaPrasad and G.Jayakrishna, " STATCOM
control under asymmetrical grid faults at fsig-based wind
farms", International Journal of Electrical and Electronics
Research, Vol. 2, No. 2, (2014), 124-132.
9. Sarrias, R., González, C., Fernández, L.M., García, C.A. and
Jurado, F., "Comparative study of the behavior of a wind farm
integrating three different facts devices", Journal of Electrical
Engineering & Technology, Vol. 9, No. 4, (2014), 1258-1268.
10. SINGH, S.K. and PRAKASH, S., "Improvement of voltage
stability and reactive power of wind farm load bus using STATCOM & sssc".
11. Angle, G., Pertl, F., Clarke, M.A. and Smith, J., "Lift
augmentation for vertical axis wind turbines", International
Journal of Engineering, Vol. 4, No. 5, (2010), 430-442.
589 E. Jamila and S. Abdelmjid / IJE TRANSACTIONS A: Basics Vol. 29, No. 4, (April 2016) 581-589
Comparative Study of the Performance of Static Synchronous
Compensator, Series Compensator and Compensator /Battery Integrated
to a Fixed Wind Turbine
REASEARCH
NOTE
E. Jamilaa, S. Abdelmjidb a Mechanical Engineering Laboratory, Faculty of Sciences and Technology FST, Road Immouzer, Fez, Morocco b Engineering, systems and applications Laboratory, National School of Applied Sciences of Fez (ENSA)
P A P E R I N F O
Paper history: Received 31 August 2015 Received in revised form 14 April 2016 Accepted 14 April 2016
Keywords: Fixed Speed Wind Turbine Static Synchronous Compensator Low Voltage Ride Through Capability Static Synchronous Series Compensator Static Synchronous Compensator /Battery
هچكيد
یک( LVRT) ییيولتاض پا قابلیت عبوز اشبهبود یبسا STATCOMبا یباتس یاًسض یساش یسهادغام ذخ ای بس زوی هطالعههقاله یيادز
یکپازچه یيب یسههقا یکگسل شبکه، یکو با استفاده اش یلدل یياست. به هواًجام گسفته سسعت ثابت هتصل به شبکه یباد یيتوزب
با هرکوز FACTS یها یستنبا س یستنس یيشده است. ا اًجام یباتس سضیاً یسهبا ذخ STATCOMو SSSC، STATCOM یساش
اش ًظس ی،باتس یکبا STATCOMدهد که یًشاى ه یجشده و ًتا یساش یهشب MATLAB/ SIMSCAPE استفاده اش ًسم افصاز
باشد. یه یيکازآهد تس ،سسعت ثابت یباد یيتوزب یک LVRTبهبود
doi: 10.5829/idosi.ije.2016.29.04a.18